Spotlight RPC: add implementations

RPC: fetchPropertiesForContext
RPC: open query
RPC: fetch query results
RPC: fetch result attributes
RPC: store attributes
RPC: close query
RPC: fetch attributenames

Also updated:
- mdssvc: add a state struct
- remove typedef slq_t, use struct sl_query instead
- fix all elements names of struct sl_query
- make a connection to Tracker
- add support functions for maintaining a list of active queries
- add hash table for query results
- add structure for ino to path mapping
- add MAX_SL_RESULTS and remove unused stuff
Remove hardcoded DBUS_SESSION_BUS_ADDRESS

A valid DBUS_SESSION_BUS_ADDRESS must be provided via the process
environment.
Fix C90 variable declarations after code
mdssvc: remove error checking macro
Spotlight: add SPAQRL mapping for kMDItemLastUsedDate